A standard tunneling apparatus has a lateral shield having a front end normally engaged longitudinally against a tunnel end face into which the tunnel is to be driven. A digging tool at the front end of the shield engages the tunnel face and is driven to dig the tunnel therein. In order to work with overpressure at the face being worked a transverse pressure wall extends across the shield and forms a pressurizable chamber inside the front end of the shield around the tool at the tunnel face. A conveyor tube longitudinally traverses this wall and has a front end open ahead of the wall and is adapted to receive material freed from the tunnel face by the digging tool. An auger in the tube is rotated to displace freed material back in the tube from the front end of the tube to the rear end thereof, which in turn is provided with a chute that receives material from the auger conveyor.
Such a machine is pushed longitudinally forward so the digging tool, which is typically a large toothed wheel, bites into the face, removing material therefrom that is conveyed back by the auger. The entire apparatus can inch forward as the face is cut away, the tunnel thus formed being concreted or lined behind the apparatus.
The overpressure at the face reduces the likelihood of cave in and holds back any ground water, in fact allowing the apparatus to tunnel through underground aquifers, under rivers, or in similarly wet surroundings. As a rule the overpressure is simply created by injecting a fluid--air, water, or a water/clay suspension--into the space within the front end of the shield in front of the transverse pressure wall. The auger conveyor is typically made of the sealing type with a very shallowly pitched auger so that the material it conveys effectively blocks the conveyor tube and prevents pressure loss back therethrough.
From the rear end of the conveyor tube the material freed by the digging tool drops through a chute onto another conveyor for transport longitudinally back out of the tunnel. A belt-type conveyor or a succession of wagons is typically used in this location behind the tunneling machine. Thus the area behind the apparatus is cluttered, making it difficult to seal the tunnel in this region. Thus either the sealing crew must work around the conveyor, the sealing work must be done relatively far behind the apparatus, or the apparatus must be shut down and the conveyor taken out of the way. All of these solutions are disadvantages.